The enthalpy (ΔH) and structural volume changes (ΔV) associated with the formation and decay of the early intermediate K600 in the photocycle of Natronobacterium pharaonis halorhodopsin (pHR), an inward-directed anion pump, were obtained by laser-induced optoacoustic spectroscopy. A large expansion is associated with K600 formation, its value depending on the medium and on the anion (Cl−, NO3−, Br−, I−). A smaller expansion is associated with K600 decay to L520. A contraction is found for the same step in the case of the azide-loaded pHR which is an efficient outward-directed proton pump. Thus, the conformational changes in L520 determine the direction and sign of charge translocation. The linear correlation between ΔH and ΔV for chloride-loaded pHR observed upon mild medium variations is attributed to enthalpy–entropy compensation effects and allows the calculation of the free-energy changes, ΔGK = (97 ± 16) kJ/mol and ΔGKL = −(2 ± 2) kJ/mol. Different from other systems, ΔS correlates negatively with ΔV in the first steps of the pHR photocycle. Thus, the space around the anion becomes larger and more rigid during each of these two steps. The photocycle quantum yield was 0.52 for chloride-pHR as measured by laser flash photolysis.
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1 September 2001
Thermodynamics of the Early Steps in the Photocycle of Natronobacterium pharaonis Halorhodopsin. Influence of Medium and of Anion Substitution
Aba Losi,
Ansgar A. Wegener,
Martin Engelhard,
Silvia E. Braslavsky
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Photochemistry and Photobiology
Vol. 74 • No. 3
September 2001
Vol. 74 • No. 3
September 2001